The present invention relates to wireless communications between devices. Some embodiments are suitable for devices one or more of which are mobile.
Wireless communications have been described in the following references:
[1] Joseph Polastre, Jason Hill, David Culler, Versatile Low Power Media Access for Wireless Sensor Networks, SenSys '04, Nov. 3-5, 2004, Baltimore, Md., USA (ACM).
[2] Wei Ye, John Heidemann, Deborah Estrin, An Energy-Efficient MAC Protocol for Wireless Sensor Networks (2002).
[3] Michael Buettner, Gary V. Yee, Eric Anderson, Richard Han, X-MAC: A Short Preamble MAC Protocol for Duty-Cycled Wireless Sensor Networks, SenSys '06, Nov. 1-3, 2006, Boulder, Colo., USA (ACM).
[4] Yu-Chee Tseng, Chih-Shun Hsu, Ten-Yueng Hsieh, Power-Saving Protocols for IEEE 802.1 1-Based Multi-Hop Ad Hoc Networks, IEEE INFOCOM 2002.
[5] Yuan Li, Wei Ye, John Heidemann, Energy and Latency Control in Low Duty Cycle MAC Protocols, USC/ISI Technical Report ISI-TR-595, August 2004.
FIG. 1 illustrates an exemplary wireless network of devices 110. Each device 110 includes a radio transceiver 120 which in turn includes a transmitter 120T and a receiver 120R that are connected to an antenna 130. The device 110 typically also includes a computer processor (processing unit) 140 and a memory 150 which stores data and computer instructions executed by processor 140. The device is powered by a power source 160, usually a battery.
A device 110 can be a single function device such as a wireless key, but may also be a multifunction device, e.g. a smart mobile phone that can make telephone calls and possibly perform other functions, such as sending emails, editing documents, and so on.
Many network topologies are available for wireless devices. One example is the conventional star topology—when the devices 110 communicate through a central device (not shown). Another example is mesh networks, including ad-hoc peer to peer networks; in an ad-hoc network any device 110 can serve as a router to pass messages between other devices.
Small size is a very desirable attribute for a mobile device, but this limits the size of battery 160 and hence limits the battery energy budget. This leads to more frequent battery replacement or recharging, which is undesirable.
The wireless radios 120/130 can be quite sophisticated, and can use complex modulation and protocols to maximize throughput in a multi-user scenario, while at the same time operating in limited available bandwidth. However, the sophisticated operation may require more operating power for the radio in transmit or receive mode.
In some protocols, to conserve battery power, the devices intermittently turn off their radio transmitters 120T and receivers 120R. These duty-cycle based communication protocols allow the radio 120 to remain in a low-power energy saving state most of the time.
In such duty-cycle based communication, there are initial discovery and synchronization operations in which the devices 110 discover each other and agree on timeslots and radio channels for communication. This synchronization or discovery protocol should also be energy efficient in order to extend battery life of the devices. For time critical application the protocol should help realize low latency in establishing communication.
Much of prior art is concerned with wireless sensor networks and the forwarding of data packets between wireless sensor nodes in a way that uses the least amount of energy. Such a network is typically a mesh network in which all nodes 110 are the same kinds of devices and communication should be possible between each pair of nodes.
As noted above, energy can be saved by duty-cycling the radio 120 of each device 110, which means that the radio 120 is not turned on all the time. In order for two devices 110 to communicate, there must be time when their radios are simultaneously on. The challenge is to find a protocol that has a low duty-cycle and at the same time low latency (that means a short time between these intersects).
One method is to have all devices 110, or groups of such devices, synchronized and have them wake up all at the same time. An example of such a protocol is presented in [5]. (The bracketed numbers refer to the references cited above.) However, time synchronization is difficult, especially when some devices 110 are movable and may enter and leave areas of network coverage.
A simple protocol that avoids time synchronization is B-MAC, in which a device 110 periodically wakes up and listens on the radio channel for a short time, and when it senses an ongoing transmission it keeps its radio 120 in receive mode until the transmission is over. FIG. 2 illustrates an exemplary timing for two devices 110, say 110.1 and 110.2. Each device 110.1 periodically turns on its receiver 120R to listen for possible transmissions. The listening periods are shown as pulses 210. When a device (such as 110.2) needs to transmit a data packet 220, the device sends the packet with a long preamble 230. Preamble 230 is long enough to be detected by any listening device. In particular, the preamble length T1 is longer than the period T2 of pulses 210. The preamble 230 contains no useful information, its only purpose is that other devices can detect an ongoing transmission and remain in receive mode. In FIG. 2, device 110.1 detects the preamble and remains in receive mode as shown at 250. When device 110.2 starts transmitting the packet 220, device 110.1 determines whether or not the packet is addressed to the device 110.1, and if so then device 110.1 receives the packet.
Another approach is that the devices 110 have a precise clock for timing such that all devices stay synchronized for a long duration, if not always. This approach is limited since it requires an affordable but accurate clock that consumes minimal operating power.
In [4], Tseng presents a “periodically-fully-awake” protocol that is illustrated in FIG. 3. Here, each device transmits beacon packets 310 at regular intervals. Each beacon packet 310 is followed by a window 312 during which data may be transferred. Every n-th beacon interval (n=3 in FIG. 3), each device has the radio in listen mode for a longer period of time to be able to receive beacon packets from other devices.